Optical proximity effect reduction methods using sub-resolution assist features have been in use for several years. Reduction methods are referred to as Optical Proximity Correction (OPC) and specific approaches include the use of scatter bars. See for instance J. F. Chen, U.S. Pat. No. 5,242,770 and sub-resolution assist features (SRAFs) as in S. Mansfield, L. Liebmann, A. Molless, and A. Wong, “Lithographic Comparison of Assist Feature Design Strategies,” Proc. SPIE 4000, pp. 63–76, 2000. Assist feature OPC has been shown to improve across pitch imaging performance of low k1 features where k1=pNA/2? and p equals pitch, NA equals the numerical aperture of the objective lens, and ? equals the exposure wavelength. Assist feature OPC has been described using various treatments but it can be most useful to consider the diffraction field effects introduced. Description and analysis has been carried out for a variety of resolution enhancement technique (RET) combinations with assist feature OPC, such as B. W. Smith, “Mutual Optimization of Resolution Enhancement Techniques”, J. of Microlithography, Microfabrication, and Microsystems, 2, 2002, and B. W. Smith, “Mutual Optimization of Resolution Enhancement Techniques”, J. of Microlithography, Microfabrication, and Microsystems, 2, 2002. For multiple assist features, bars are evenly spaced within a space opening between main features. Bars are sub-resolution and the bar frequency is generally beyond the diffraction limits of the imaging system. Because of this, no first order diffraction energy is collected from the bars making the bar frequency inconsequential. As an example, consider an imaging situation for 150 nm main features with a 1:5 duty ratio using 248 nm wavelength and a 0.70 NA objective lens. A typical bar size may be 60 nm and three evenly spaced bars can be inserted between features with a bar pitch of 187.5 nm. The resulting k1 for the bars is 0.27, effectively eliminating lens capture of first diffraction orders using σ values of 0.95 and below. With only zero diffraction order collection, the entire space between the main features experiences a reduction in intensity as a function of the bar width (b) and bar pitch (pb):
                              Space          ⁢                                          ⁢          intensity          ⁢                                          ⁢          reduction                =                                            (                                                                    p                    b                                    -                  b                                                  p                  b                                            )                        2                    =                                                    (                0.68                )                            2                        =            0.46                                              (        1        )            
The result is exactly that which would be expected if the space transmission was equivalently reduced. It has been suggested that the effect of the adding multiple assist bars corresponds to the introduction of frequency terms to isolated features so as to resemble that of the dense features. This analysis is problematic on two accounts. First, as shown above, the frequency of the bars is often beyond imaging limits, eliminating all but their zero diffraction order influence. Second, if the bars are placed at a frequency that matches that of the dense main features, the likelihood that the bars will print increases when using modified illumination. The frequency of the bars would be such that off-axis distribution of diffraction energy will increase the modulation and the depth of focus of the bars themselves.
Current methods used for OPC layout are based on sets of rules for best pattern performance (known as rules-based OPC) or the extraction of models based on the parameters that result in best pattern performance (known as model-based OPC). With rules-based approaches, adjustments are applied to feature edges, bar sizes, and spacing based on pattern width and edge location. Model-based approaches are based on the convolution of predefined kernel functions with a mask function. See for example J. P. Stirniman, M. L. Rieger, “Fast Proximity Correction with Zone Sampling,” Proc. SPIE 2197, pp. 294–376, 1994. Kernel functions may incorporate specific optical, resist, and etch behavior. Both methods are concerned with the spatial (X-Y) fidelity between the structure layout, the mask function, the intensity images, and the images formed in photoresist. It is difficult to optimize lithographic imaging parameters such as OPC strictly from the traditional standpoint of dimensional analysis. Furthermore, it is difficult to perform optimum OPC layout using models or rules based on spatial (or X-Y) information and performance. Such approaches usually lead to sets of rules that are only valid around a small factor sample space. Furthermore, the current approaches do not address the unique nature of a masking layer that consists of a series of contact features or vias.
In order to connect a one level of a device to another, the device must have an array of contacts. Some devices have many contacts and the pattern of a contact mask may have a number of dense features and semi-isolated features. It is desirable to fabricate devices with about the same size contacts. However, often the density of contact features will enlarge contacts in the dense region and contacts in a semi-isolated region will be reduced in size. The disparities in sizes of contact openings or vias is undesired and there is need to correct this problem and provide a lithographic technique that will provide local correction of contact mask to provide uniform contact features.
At present, the rule based systems and model based systems and combinations of the two are complex and often interfere with one another. In other words, corrections made by the rule based systems may offset corrections provided by the model based system.